The invention relates to a cylinder head of an internal combustion engine.
German Published Patent Application No. 39 15 988 describes a cylinder head of an internal combustion engine, which has at least one exhaust port, which is formed by a sheet metal port cast into the cylinder head. The sheet metal port is composed of at least two layers of sheet metal, which form a gap, which constitutes a heat-insulating layer. A port cast or inserted into the cylinder head of an internal combustion engine in order to thermally insulate the hot exhaust gas from the cylinder head material and the coolant is known as a port liner.
An object of the present invention is to provide a multilayer sheet metal port liner for an exhaust port in the cylinder head of an internal combustion engine, which while affording improved heat insulation, is easy to manufacture.
In the cylinder head of an internal combustion engine according to the present invention with a cast-in port liner of at least two layers, provided with a heat-insulating layer and having a sheet metal port, the sheet metal port is helically wrapped by a sheet metal strip, a heat-insulating layer arranged both between the individual layers of the sheet metal strip and also between the sheet metal strip and the sheet metal port. Winding a sheet metal strip around a sheet metal port makes a multilayer port liner, provided with an insulating layer, for an exhaust port in a cylinder head of an internal combustion engine easy and inexpensive to manufacture.
In an example embodiment, the sheet metal port is cast in the cylinder head in the area of the valve guide. In order to prevent any distortion of the port liner as a result of the large temperature fluctuations in the operation of the internal combustion engine, the port liner is firmly cast in the cylinder head at only one point. This may be done in the area of the valve guide, since due to an approximately central fixing of the sheet metal port, the thermal expansion occurs in two directions. As a result, the absolute thermal expansion at each of the two port ends is less than if it were fixed at one end.
In another example embodiment of the present invention, the sheet metal port is supported by the wound sheet metal strip so that the port is longitudinally displaceable in the cylinder head, in order to prevent any deformation due to the stresses in the event of high thermal expansion. The sheet metal port is able to slide longitudinally in the wound sheet metal strip due to the single support for the sheet metal port in the area of the valve guide and because the sheet metal strip is cast into the cylinder head.
In a further example embodiment of the present invention, the sheet metal strip is provided with a coating. This makes the sheet metal strip easy to deform when winding and at the same time affords a heat-insulating effect.
The coating may be made of graphite. When casting, graphite does not release any gases, which might lead to the formation of shrinkage cavities in the cylinder head material. Graphite is furthermore a good thermal insulator.
The sheet metal port may be manufactured by internal high-pressure metal forming. By this method, it is possible to manufacture a sheet metal port of virtually any form and shape without a joint weld seam or similar flow restrictions.
In a further development of the present invention, the sheet metal port has an essentially greater wall thickness than the sheet metal strip. The wall thickness of the sheet metal port is approximately 1 to 1.5 mm, for example. The sheet metal port thereby has a low heat capacity, which may allow rapid heating up of the exhaust gas. It nevertheless may allow adequate strength at high exhaust gas temperature and exhaust gas pressures. The sheet metal strip is made, for example, from a steel foil a few tenths of a millimeter thick. Such a thin foil may be made to conform to a given shape of the sheet metal port by helically winding and where necessary pressing on. The sheet metal strip in conjunction with the coating affords good heat insulation for adequate strength and service life of the overall port liner.
One example method of manufacture is to wrap a tube with sheet metal strip, and then to form the wrapped tube into a sheet metal port by internal high-pressure metal forming. Since a straight tube is much easier to wrap than a pre-formed sheet metal port, such a method of manufacture is well suited to automated wrapping.
Further features and combinations of features are set out in the description and the drawings. Exemplary embodiments of the present invention are represented in simplified form in the drawings and explained in more detail in the following description.